HELP! - iMovie Freezing After Every Click/Action

Hi. I am running a 2012 15" MacBook Pro with: i7 Processor, 16 GB RAM, 500 GB HDD (412 GB Free), NVIDIA GeForce and Intel Graphics cards... Running macOS High Sierra and using iMovie 10.1.9.

I am trying to edit some HD video footage. I have always used iMovie and Final Cut Pro to do my editing but have run into some major issues with the new versions. First, I couldn't import video without my RAM usage spiking to more RAM than my system actually had at which point the system would lock up and a restart was necessary to regain full use of the system. Restarts took a long time as well. I finally decided just to do a full wipe and reinstall High Sierra and iMovie... the system literally has only High Sierra and iMovie installed.

So now import works but now previews only load for portions of the imported videos and moving the cursor from one point in a video to another and clicking causes the rainbow beachball of death to appear for minutes at a time and iMovie to show as Not Responding in Activity Monitor while also showing over 100% CPU usage. Eventually, it recovers but I can't edit video when I have to wait ten minutes between clicks.

Any advice on how to fix this issue would be greatly appreciated. Thank you in advance!